A conventional example of a backlight device used in a liquid crystal display device and the like includes cold-cathode tubes, and a diffusion plate disposed in the direction in which the cold-cathode tubes emit light. The diffusion plate is printed with a dimming dot pattern that has a total light transmittance of 62% to 71% and a haze value of 90% to 99%. The dimming dot pattern includes dots that are arranged at positions dividing the interval of the cold-cathode tubes into 24 parts to 48 parts. The diameter of the dots varies from 0.16 mm to 0.7 mm in accordance with the distance from the cold-cathode tubes. Such a backlight device is described in Japanese Unexamined Patent Application Publication No. 2005-117023.
In the backlight device described in Japanese Unexamined Patent Application Publication No. 2005-117023, the dimming dot pattern has a regular and point-symmetric distribution on a plate surface of diffusion plate. Accordingly, the light emitted from the cold-cathode tubs has a simple luminance distribution due to the point-symmetric dimming dot pattern. As a result, light-dark unevenness (luminance unevenness) has tended to become readily visually recognizable. In particular, when the distance between the light source and the diffusion plate is reduced, or the diffusion plate is made thinner in order to achieve a decrease in thickness, luminance unevenness has become more likely to be visible.